1. Field of the Invention
The present invention relates to a friction transmission unit for transmitting rotational force of an input member to an output member by means of friction force, and in particular to the profile of the contact portions of the input and output members.
2. Description of the Related Art
In known friction transmission units, power is transmitted from one to another of two members which are in rolling contact by means of friction force caused between the contact portions of the two members. The friction force is substantially proportional to a normal force acting on the contact surfaces of the two members. That is, the larger the force to press the two members together, the larger the power to be transmitted. Meanwhile, the associated increased contact stress caused by the greater force pressing the two members together may cause wearing-out at or below the surfaces of the members. The wearing-out can be reduced by employing a larger contact area thereby reducing contact stress. However, when the relative velocity between the two members within the contact region is not constant, the difference between the velocity increases, causing problems such as wearing-out, heat generation, and reduction of transmission efficiency.
Thus, in a structure in which constant relative velocity is not obtained within the contact region, a smaller contact region and smaller contact stress are preferable. These can be achieved using a known method for attaining constant stress distribution in the contact region. An example profile of the contact portions of the two members which satisfy this condition is disclosed in Japanese Patent Laid-open Publication No. Hei 10-89431, which suggests a contact portion having a Lundberg's profile.
The Lundberg's profile, as disclosed in the above-mentioned application, however, has a problem that it does not, in fact, attain constant stress distribution and, thus, does not realize sufficiently significant reduction of contact stress. Lundberg's profile has another problem that stress peaks in areas near the edges of the contact region, where the relative velocity is relatively high.
Further, should the portion of at least one of the input and output members be changed, conditions of the contact region also changes. The above-cited application does not disclose information concerning design conditions, including a most preferable position wherein the profile of a contact portion should be optimized.